As one Grand National horse at the back is well-groomed (6)
I believe the answer is:
soigne
'well-groomed' is the definition.
(synonyms)
'as one grand national horse at the back' is the wordplay.
'as' becomes 'so' (I've seen this in other clues).
'one' becomes 'i' (Roman numeral).
'grand' becomes 'g' (G is short for grand as in a thousand).
'national' becomes 'n' (abbreviation listed in Chambers).
'at the back' says to take the final letters (the letter at the back).
The last letter of 'horse' is 'e'.
'so'+'i'+'g'+'n'+'e'='SOIGNE'
'is' is the link.
(Another definition for soigne that I've seen is " well-groomed".)